The present invention is directed to an interface between two electrical circuits operated at different operating voltages. An input signal of the interface corresponds to an output signal of the first circuit, the input signal being compared in a comparator to a reference voltage which is proportional to a first operating voltage. An output signal of the comparator corresponding to an input signal of the second circuit.
Such interfaces are required when circuits having different operating voltages or different signal levels are to interface with one another, for example, circuits used in motor vehicles. In motor vehicles signals are generated in circuits operating from a battery voltage UB of the onboard network, the output signals are processed in further circuits that operate from a different supply voltage UV. A disadvantage of this system is that the battery voltage UB and, thus, the output level of the circuit operated therefrom fluctuates greatly and can have considerable noise signals.
In the worst case, an improved signal-to-noise characteristic can be achieved when the input level of the receiving circuit is kept proportional to the operating voltage of the transmitting circuit. This is usually achieved by a reference voltage UR for an integrated comparator of the receiving circuit being derived from a voltage divider from the operating voltage of the transmitting circuit, i.e. from the battery voltage for a motor vehicle. The output signal of the transmitting circuit is compared to this reference voltage UR. The output signal of the comparator that is at the supply voltage of the receiving circuit is independent of the magnitude of its input signals. The drawback of this solution is that an integrated circuit is used that is extremely expensive and that requires a great deal of space.